1. FIELD OF THE INVENTION
The present invention relates to turbidity meters which measure the clarity of water in an oceanic or like environment.
2. DESCRIPTION OF THE PRIOR ART
Many of the recent studies of water quality, for example in various salt water bays, have emphasized the importance of water clarity among the indices describing the condition of the water. Turbidity of the water is a term generally applied to the clarity of the water, and, more specifically, to the cloudy or hazy appearance in a naturally clear liquid caused by a suspension of, for example, fine particles. However, in these studies almost all descriptions of water turbidity in such bodies of water are approximate and anecdotal in nature. Detailed qualitative information on turbidity seems to be lacking in most other studies of water quality in the coastal environment, as well.
One reason for this lack of information on what is certainly an important parameter in determining water quality, is the lack of a turbidity meter in the prior art which can be left in the body of water being monitored for a substantial period of time to record the information. Secondly, turbidity meter designs of the prior art have used complex systems of baffles or shutters to eliminate the effects of daylight interference. More specifically, the irradiating beams of the prior art devices normally comprise electromagnetic radiation having a wavelength in visible light spectrum. Hence, daylight interferes with scatter light detection, such detection being commonly used in these prior art turbidity meters. Therefore, a practical, functional turbidity meter does not exist in the prior art.
A practical, functional turbidity meter placed in strategic positions in an estuarine environment can provide invaluable data on the behavior of turbidity under various wind and tide conditions. In this way, a systematic approach to the question of water clarity in an environment, such as a bay, may be undertaken. Such a meter is also very attractive for use in monitoring operations such as dredging or filling, in order to make certain that environmental requirements are not being exceeded. However, the prior art has not provided a turbidity meter capable of achieving these objectives.
There are many discussions of turbidity in the literature, including both the effects of absorption and the scattering mainly due to particles suspended in the water. As has been pointed out, however, the principal problem in seeing through seawater is usually scattering by particles. The total scattering coefficient in the sea includes the effect of the water itself, dissolved material, and suspended particles. In any but the clearest water, however, the main contribution to the scattering coefficient is due to the suspended particles. If a volume V of water is irradiated by a beam of light of wavelength .lambda., then the intensity, I, of the light scattered into direction .theta. measured from the direction of the beam is given by ##EQU1## where .beta. (.theta., .lambda.) is the scattering function, which depends on both wavelength and scattering angle, and E is the incident irradiance into the volume element dv. This means that to obtain an exact determination of .beta., measurements must be made at all angles and all wavelengths. However, by comparing a portion of the scattering at a given angle to the total scattered light in all directions, it has been shown that for typical oceanic waters, measurements made at, for example 45.degree., will yield accurate values of the total scatterance within a few percent. This greatly simplifies the practical problem of measuring turbidity. In addition, it has been shown that the wavelength dependence of scattering from particles in the sea (most of which are larger than a micron in diameter) is very small.
The prior art may be illustrated by U.S. Pat. No. 3,278,753, to Pitts, showing an underwater detection system wherein light backscatter is utilized to detect the presence of an object. U.S. Pat. No. 3,406,289, to Schleusener, illustrates the use of an infrared laser in a cavity containing a gaseous medium.